1. Field of the Invention
The present invention relates to the manufacture of glass strands which are obtained by a mechanical drawing process and which are in particular intended for reinforcing thermoplastic or thermosetting organic materials or cement-based mixtures.
2. Discussion of the Background
Continuous glass strands are obtained by mechanically drawing a large number of thin streams of molten glass flowing out of apertures in the base of a die plate in the form of continuous filaments. During the drawing process these filaments, distributed in one or more separate layers, are coated with a sizing composition by a suitable device before being collected in order to produce one or more strands. This sizing composition, which is to impart different characteristics to the glass strand, must firstly have properties imposed by the glass fiber manufacturing process. In particular it must be stable and preserve a constant composition, in spite of the shearing forces permanently caused within the sizing composition by the filaments passing therethrough at speeds of the order of several tens of meters per second.
This sizing composition then has to protect the filaments from abrasion caused by the unavoidable rubbing of the strand against various surfaces. Generally it also has to ensure the integrity of the strand, i.e., the interconnection of the filaments constituting the strand. This characteristic is important since it not only enables a strand to be easily extracted from a wound package (a form in which glass strands are frequently found) but it also restricts the number of filaments which rub directly against all the fiber-guiding devices. If the strand is to reinforce an organic substrate, the sizing composition must also facilitate the wetting by this substrate of the filaments constituting the strand.
Finally, if the glass strand is to reinforce organic substrates, the sizing composition deposited on the filaments must be chemically compatible with these substrates.
Almost all the known sizing compositions are sizing compositions in the aqueous phase which are in the form of solutions, or, far more frequently, in the form of a suspension or emulsions in water.
Before being combined with an organic substrate, the glass strands sized in this manner have to be dried in order to eliminate the water which becomes useless or even a hindrance. To this end, the wound packages are subjected to temperature cycles generally of between 110 and 150.degree. C. for periods of time which can reach and exceed 16 hours. In effect, the amount of water to be evaporated is large since, on average, it represents at least 10 to 15 weight % of the wound package. This operation requires particular installations and a level of energy consumption of which the cost puts a strain on the production costs.
In addition, the drying of the wound packages can sometimes impair the quality of the strand when it causes irregular migration of the sizing composition constituents through the turns of the wound packages.
In order to overcome these disadvantages, it is known to dry directly the strand (U.S. Pat. No. 3,853,605) or the filaments (WO-92/05122) before the wound package is formed.
Drying the strands during the fiber-drawing operation requires the installation of devices such as ovens below each die-plate. Moreover, the efficiency of this drying method is closely linked to the manufacturing conditions such as the amount of water on the strand, the composition of the sizing composition, the drawing speed, and the number and diameter of the filaments, etc.
For the rare cases of sizing compositions composed solely of organic constituents, the glass strands coated with these sizing compositions are frequently subjected to a particular treatment before they are wound onto a rotating support. The object of this treatment is to modify the sizing composition of the strand before it is wound such that the sizing composition does not cause the turns of the wound package to adhere to one another too intensively. This adherence is liable to render the unwinding of the strand difficult, if not impossible. This treatment consists, for example, in heating the layer of filaments coated with sizing composition so as to eliminate the solvent from the sizing composition before the strand is formed as a result of the collection of the filaments (U.S. Pat. No. 3,425,862) or in polymerizing the sizing composition by subjecting the strand to the action of ultraviolet radiation (U.S. Pat. No. 5,049,407) over at least a part of its path. Here again, this type of treatment requires the installation of devices below each die plate and does not prevent the search for the best possible compromise between the integrity of the strand and its ability to be wetted by an organic substrate.
An integral strand, coated with a purely organic sizing composition, can be produced without it having to be subjected either to a heat treatment or any other treatment; it is the sizing composition alone which connects the filaments to one another. U.S. Pat. No. 4,530,860 describes sizing compositions of this type which comprise a high percentage of film-forming agent of which the essential role is to impart the integrity necessary for handling the strand.
The object of the present invention is to provide a process for manufacturing a glass strand coated with a sizing composition such that it permits the drying operation and the resultant disadvantages to be avoided.